The rough terrain crane (also called a wheeled crane) as one kind of self-traveling working machines has conveniences capable of small sharp turns and capable of moving quickly, and therefore the crane has been used also in a small scaled construction site for construction of a private house or the like. Further, a residential district is so narrow in road width that the crane need to move into a narrow section passing through the roads provided with obstacles such as electric poles, street lamps, trees and the like, and therefore various devices have been made to narrow the width of the machine.
As a rough terrain crane having the width of the machine reduced, the crane disclosed, for example, in Japanese Patent Publication No. Hei 6-39316 is well known.
In this crane, a multiple telescopic boom as a working attachment is offset toward one side from the center of the machine body in order to narrow the width of the machine while securing a space of a cabin.
However, in the boom of the rough terrain crane, a rate of weight held with respect to the weight of the machine body is very large, about ⅓, and therefore such a large-weight boom as described is offset, the center of gravity of the machine body of the whole rough terrain crane is deviated from the center in the width direction of the machine body. Accordingly, the machine body becomes unbalanced in terms of weight to the left and right, and therefore a tilting angle defined in the vehicle control rule in Japan cannot be secured as the case may be, failing to clear car inspection, thus producing an evil that cannot run on a general public road as a vehicle.
Further, conventional self-traveling working machines including a self-traveling crane had the following problems.
A. Problem on the center of gravity of the machine body A description will be made of a self-traveling crane disclosed in Japanese Patent Laid-Open No. Hei 9-39645 Publication, as an example (see FIGS. 27-29).
This self-traveling crane is that a position of the center of gravity of the machine body is lowered, and a frame (a traveling frame) of a lower traveling body is made to have a high rigidity, thereby enhancing the ability of the crane.
That is, in a self-traveling crane having a crane apparatus mounted on a traveling frame 72 of a lower traveling body 71, the traveling frame 72 is formed to be box-like whose section is almost closed by two side plates 72c, 72c opposed to each other by connecting an upper plate 72a, a bottom plate 72b, and an upper plate 72a, a bottom plate 72b. 
Further, front and rear drive shafts (hereinafter referred to as an axle) 74, 74 pass through the traveling frame 72 through through-holes 73, 73 provided in the side plates 72c, 72c of the traveling fame 72, and drive shafts 75, 75 for transmitting power to differential gears of the axles 74, 74 also pass through the traveling frame 72.
A rotating pedestal portion or a rotating base 76 is disposed in a center portion in the width direction of the traveling frame 72, and an upper rotating body 79 provided with a multistage telescopic rising/falling boom 77 as a working attachment and a cabin 78 is mounted on the rotating pedestal portion 76.
An engine 80 is mounted at the rear of the traveling frame 72.
The following effects can be obtained by employing the above-described structure for the traveling frame 72 of the self-traveling crane.
{circle around (1)} By passing the axle 74 through the traveling frame 72, the position of the traveling frame 72 lowers to lower the height to a roof of the cabin 78, that is, lower the height of the crane in its traveling attitude.
Because of this, since the position of the center of gravity of the whole crane lowers, the tilting angle becomes large, and the stability during crane operation enhances. Further, the position of the center of gravity of the crane lowers to thereby provide an allowance in the vertical direction. That is, the height of the traveling frame 72 itself can be made high, and a sectional area of the traveling frame 72 can be made large, thus enhancing the strength thereof.
However, according to the above-described crane, the rotating pedestal portion 76 for supporting the upper rotating body 79 rotatably is disposed on the upper surface of the traveling frame 72 and is at a position above the support position of the front and rear axles 74, 74, which is therefore insufficient in terms of lowering the height of the machine, and particularly, smaller the width of the machine, it is difficult to secure the traveling stability.
B. Problem on the length of the working attachment
In the self-traveling working machine including a self-traveling crane, the upper rotating body is rotatably mounted on the lower traveling body provided with wheels or crawlers, and the cabin and the working attachment are provided on the rotating frame of the upper rotating body.
In the case of the crane, as the working attachment, the multistage telescopic boom is generally supported free to rise and fall on the rotating frame and mounted from backward to forward of the rotating frame passing through one side of the cabin in the maximum fallen state.
Incidentally, the lengthy working attachment is sometimes required in a narrow depending on the using conditions of the machine, but in mounting the lengthy working attachment in the self-traveling working machine, there results in the multistage system in order to secure the performance at the time of normal traveling time and the workability in a narrow, and the longer the length, the number of stages of the working attachment increases to increase the weight thereof.
For solving such a problem as described above, in the mobile crane shown, for example, in Japanese Utility Model Registration No 2529509 Publication, the base end of the internal boom is projected rearward from the base end of the external boom in the state that the telescopic boom is most contracted, whereby a pivotal connecting position (a rising/falling position) of the external boom can be moved in the direction of the end of the telescopic boom by a length portion corresponding to the rearward projecting amount of the internal boom.
As a result, the telescoping stroke of the internal boom with respect to the external boom can be increased without increasing the rotating radius of the rear end of the upper rotating body when the crane is working.
Further, in the crane proposed in Japanese Patent Laid-Open No. Hei 3-211193 Publication, the boom is rotatably pivoted at the end of the base boom, the base boom is made to be rocked laterally with respect to the boom base portion, and even if the lengthy boom is arranged to meet a condition, namely, the radius of rear end of boom at the time of travel>radius of rear end of frame>radius of rear end of boom at the time of working, the safety at the time of traveling is secured, and the rotating radius of the rear end when the crane is working is made small to enhance the workability at a narrow.
Further, in the anchor executing machine proposed in Japanese Patent Laid-Open No. 2001-140575 Publication, the boom bracket can be moved laterally with respect to the base frame to thereby enhance freedom of execution.
However, in the art shown in the aforementioned Japanese Utility Model Registration No. 2529509 Publication, the internal boom is movably arranged within the external boom so that the internal boom is projected from the rearward of the external boom, and therefore the full length of the external boom is unavoidably shortened. Accordingly, there poses a problem that the number of stages of the telescopic boom increases naturally, and the weight of the telescopic boom increases.
Further, a trouble occurs in rising/falling rocking of the telescopic boom unless the internal boom is moved in an extending direction with respect to the external boom, and therefore a clearance is provided at the lower portion of the telescopic boom, which poses a further problem that a position of the center of gravity of the telescopic boom is high. There is possibility of impairing the stability of the mobile crane during traveling.
On the other hand, in the art shown in Japanese Patent Laid-Open No. Hei 3-211193, the base frame and the boom constitute an articulated boom, therefore posing a problem that construction becomes complicated and the weight increases. Further, there poses a further problem that the height of the rear end portion of the boom is high, and a position of the center of gravity of the boom is high, and the stability of the machine body during traveling is possibly impaired.
Further, in the anchor executing machine shown in Japanese Patent Laid-Open No. 2001-140575, the boom bracket itself can be moved laterally, with respect to the structures of the mobile crane shown in Japanese Utility Model Registration No 2529509 Publication, and the crane shown in Japanese Patent Laid-Open No. Hei 3-211193, but the boom bracket can be moved only on the upper rotating body, thus posing a problem that when a lengthy boom is mounted, the projecting amount forward of the upper rotating body increases or the number of stages of the boom increases, resulting in an increase of movement into a narrow or of weight. Further, in the anchor execution machine, since the boom is pivoted at a high position, the center of gravity of the machine body is also high.
With respect to the self-traveling working machine which runs on the road, there are many problems such as a problem of a weight limit of road, and, there is disadvantageous in terms of the travel performance such as acceleration, braking or turning due to an increase in inertia force during traveling resulting from an increase in weight, and the traveling apparatus for supporting a great weight is unavoidably large-scaled, resulting in an increase in weight by which the entry into a narrow is lost. Further, the higher the center of gravity of the machine body, there is disadvantageous in terms of traveling stability with respect to the direction of falling down toward the side.